Here's a good tip that I thought was worth sharing: Neil from AppScout responds to a user's question about a message he gets when opening Internet Explorer 7: "Message from hook.dll. Can't create File Mapping."
Hook.dll? What the heck is that?
What the problem hints at is an issue with an Internet Explorer add-on, and one with a unimaginatively named system hook - hook.dll. Here's how to resolve:
Right-click the IE7 icon and choose "Start Without Add-ons." If the problem doesn't reoccur, then you know that it's related to some IE add-on. To figure out which one, reopen IE normally and choose "Manage Add-ons" from the "Tools" menu. Choose "Enable or Disable add-ons" and then disable all but one. Reopen IE. Still no issue? Keeping enabling add-ons one by one (restarting IE each time) to determine the culprit.
If you're on Vista, and you don't see the right-click option to "Start without Add-ons," then go to Start, click All Programs, click Accessories, click System Tools, and then click Internet Explorer (No Add-ons).